Arc discharge and method of producing the same



March 15, 1960 R. v. NEIDIGH ARC DISCHARGE AND METHOD OF PRODUCING THE SAME Filed July 9, 1958 3 Sheets-Sheet 1 INVENTOR. Rodger V. Neidigh ATTORNEY March 15, 1960 R. v. NE IDIGH 2,928,966

ARC DISCHARGE AND METHOD OF PRODUCING THE SAME Filed July 9, 1958 3 Sheets-Sheet 5 CENTRAL ARC RIBBON Fig. 3.

INVENTOR. Rodger V. Neidigh ATTORNEY United States Patent C ARC DISCHARGE AND METHOD OF PRODUCING THE SAME Rodger V; -Neidigh, Knoxville, Tenn., assignor to the UnitedStates of America as represented. by the United States Atomic Energy Commission .Application July 9,1958, Serial No. 747,551

4 Claims. or. 313-63) This-invention relates to a method of producing an arc d scharge which is magnetically-collimated and wherein the. electrons are very energetic andtransfer this energy to'rons, thereby causing the ions to be accelerated either- 10* mm. Hg and is nearly uniform throughout the- If an insulatedanode is used, the

length of the arc. potential thereof is about the same .as the cathode and the. anode is subject to intense heating. Typical of such arcs. are the arcs used in calutron source-units. and many spectrographic analysis equipments. It has been discovered that'ifthe; pressure in the region ofthe. arc is reduced.whilemaintaining a constant rate of gas-feed, the original arc will undergo an abrupt change in appearance and characteristics. This change occurs in the pressure range of 10" to 107 mm. Hg. The brightness of the arc decreases and a luminous plasma appears surrounding the new arc. The insulated terminal electrode is heated only slightlyby this new arc, in contrast to the severe heating. of. the anode inthe previously described conventional arc, and, unexpectedly, the potential thereof becomes much more negative than' the cathode. Another characteristic feature of the are described herein is the existence of'a pressure gradient with ranges from about mm. Hg near the point of. gas feed to the arc, to about 10- mm. Hg in the volume surroundingthe main body of the arc. One outstanding characteristic of this new type of: arc is the high efiiciency of energy transfer from electrons to the ions formed in the arc,,with greater than 90% of the input gas being converted into energetic ions. 2 Also, the electrons accelerated from the heated filament possess an energy spread much greater than inthe usual arc and the resultant ions are relatively energetic. acteristic of this-arc whereby ions are accelerated. (without' physical electrodes) with a radial velocity component; in fact, the greater portion of their energy is in a direction perpendicular to the magnetic field. Accordingly, it is an object of this invention to provide an arc wherein the electrons are very energetic. It is another object of this invention to provide highly energetic ions by collisions with electrons which are accelerated to relatively high'energies within the arc.

These and other objects and advantages will become apparent from a consideration of' the following detailed specifications and the accompanying drawings wherein: Fig. 1 is a schematic drawing showing apparatus for the production of an energetic arc in a unifc'nn magnetic field.

- Fig. visa-schematic drawingshowing apparatus for 2,928,966 Patented Mar. 15,1960

the use of the energetic arc in a magnetic-mirror type of device, and

Fig. 3 is a plot of equipotential lines in the energetic. are as determined by floating-probe measurements.

In Fig. 1, a uniform magnetic field is provided by magnets 8 and 9. A negatively biased tungsten filament 1, which is heated indirectly or by ion bombardment is. disposed on an axis substantially paralleled to the magnetic field. A typical heating circuit is shown by the battery 47, lead 46, heater 44, lead 45 and back to battery 47. A floating electrode or electron reflector 2' is'provided, and electrons, produced by the heated'filament, travel parallel to.the magnetic fieldthrough a pair of grounded, apertured collimating bafiles 3 and 4, and strike the electrode 2. The baffle 3 is grounded through lead 42. Baflle 4 is grounded through lead 41-. Electrode 2 is mounted by meansv of electrical insulator 38. Gas is fed from a source 6 through feed tube 5 to the region in the collimating aperture of baffle 3 nearest the filament 1. The chamber 10 is evacuated to a pres sure described hereinafter. With a negative bias (-150 volts, for example, from ground through battery 49 and lead 48) applied to the filament 1 and a gas feed such that the pressure near the filament is about 10- mm. Hg, an arc is struck. The electron sheath 11 extends.

. very near to the floating electrode; however, the arc. 7 plasma 7 extends only a. portion of the distance torthe At least equally important is the unique charsecond collimating bafile 4 when the gas pressure in the I outer regions is maintained at about 10- mm. Hg. A vacuum gage 10' is connected tothe chamber 10-for readily determining the pressure within said chamber. The potential of the floating electrode 2 unexpectedly achieves. a value at least twice the bias applied to the filament during arc operation. p

In one embodiment of the apparatus shown' in Fig. l', the baflles 3 and 4 are made of one-half inch copper, and are approximately six inches square. The circular holes in these baffles through which the arc passes are approximately one-fourth inch in diameter. four inch oil diflusion pump may be used for evacuation of chamber 10. Arcs have been operated in uniform magnetic fields of 2000-7000 gauss. The maximum cathode bias used'was 3.5 kv..at 8 amperes. Deuterium, hydrogen, nitrogen and most of the noble gaseshave been utilizedas feed gases.v The maximumdeuterium ion energy observed has been 2.5 kV., while 5 kv. has been observed for ions of the heavier gases.

Some characteristics of the arc discharge arezfi'r'st, positive ions are shot out of the region of the arc centerline, with velocity components across the field and to.- ward the baflle plate 4; second, the energies associatedwith such ions can amount to as much as twice the numerical equivalent of the potential placed on the hotcathode; and third, a discharge is setup to one side of the arc centerline and rotates about the arc centerline, having arough wedge-shape cross-section and ex: tending between the baffies. The first characteristic mentioned above can be determined by sputtering patterns, Faraday cup collector signals, and doppler slant effects observedspectroscopically. The second characteristic above can be determined by sputtering patterns and collector currents obtained by use of a simplified mass spectrograph for a given gas and magnetic field. The

' third characteristic above can be determined byv theme of a floating probe, such as shown in Fig. 2; disposed in the region adjacent to the arc. The rotating discharge is always in the direction a positive ion would take, and

the speed of rotation is in the range of 20,000to 100,000

floating probe. Floating probe measurements have been A twenty;

made of the potential in the plasma surrounding the central arc column. An instantaneous plot of this potential is shown in Fig. 3. 'It is possible to formulate a tentative model of the, rotating plasma effect external to the arc column. Thus at the onset of the are discharge, positiveions are-sprayed out into the region surfounding the column. Some of these ions charge-exchange with the neutrals in the region, forming slow ions, and thus a positive potential is built up. The ions that do not charge-exchange strike the baflle plate 4, causing sputtering and giving rise to free electrons. Thus the increasing potential and available electrons eventually cause a gas discharge along the magnetic field at some point away from the central column. The discharge now establishes itself in some mode compatible with the givenboundary conditions. It cannot remain still, since it has destroyed the potential that established it. However, examination of Fig. 3 shows that a radial electric field directed outward is associated with the high potential. The numbers in Fig. 3 indicate positive potentials with respect to the end bafiles. These are typical of the potentials assumed by a floating probe at any instant for given operating conditions. The field rotates as a unit about the arc in the direction indicated. The direction of the magnetic field H is into the paper. Referring now to Fig. 1, when electrons are accelerated into the high potential region from the end baflle, they are shifted in the direction a positive ion would rotate (counterclockwise) by the effect of the electric field crossed with the magnetic field. Thus propagation of the discharge is favored in the direction of this shift, due to the ionizing effect of the shifted electrons at the edge of the discharge. The over-all picture of the rotating discharge is then as follows: The discharge always rotates into a high potential region, and leaves behind it a region of zero potential. However, the continuous spray of ions rebuilds this potential at a steady rate, thus maintaining the high potential region toward which the discharge moves. The speed of rotation is governed by the time necessary for the potential to rebuild at any one point, and thus is a function of system parameters.

7 As discussed above, the potential of the floating elec trode 2 reaches a value at least twice the biasapplied to the filament 1 during arc operation. A portion of the ions are accelerated to energies numerically equivalent to this floating electrode potential by time-varying electric fields which exist in the central arc region. Ion energies increase linearly with cathode bias, and the perpendicular component of the ion energy is comparable with and increases with increased floating electrode potential. This acceleration of ions, is always associated with the pressure gradient in the are column and occurs only when electrons are contained by the potentials at the ends of the arc, and when the loss of ions is restricted by the extra large sheath dimension imposed by the low pressure. An increase in the pressure gradient is also known to increase the ratio of the floating electrode potential to the cathode potential.

The time-varying electric fields which cause the radial ion acceleration are, in fact, related to the rotational effect discussed above with reference to Fig. 3.

, The device set forth in Fig. 2 shows the use of the arc of Fig. 1 in a mirror type device. Electrons are produced by a heated filament 12 which is heated by a heater 31 connected to a battery 34 by leads 32 and 33, and-travel in a path as determined by an applied magnetic field formed by magnets 18 and 19 and a hollow iron cylinder 21 disposed between the magnets. The field configuration between the magnets 18, 19 and iron cylinder 21 is shown by the dotted lines in the figure. Magnetic field strengths at the ends of the apparatus are approximately twice those within the central portion of the apparatus. The electrons pass through an apertured grid 20 and through apair of grounded, apertured collimating bafiles 14, 15, and strike an insulated (floating) electron-reflector electrode 13. The purpose of grid 20 is to pulse the are by a high negative bias so that plasma decay times could be measured. The baffle 14 is grounded by lead 29. Bafile 15 is grounded by lead 28 and electrode 13 is mounted on electrical insulator 26. Gas is fed from a source 17, through a feed tube 16 to the region in the collimating aperture of battle 14 nearest the filament 12, and the chamber 24 is evacuated. With a negative bias volts, for example, from ground through battery 36, and lead 35) applied to the filament 12, and a gas feed such that the pressure near the filament is about 10 mm. Hg, an arc is struck. The electron sheath extends nearly to the floating electrode 13, however, the arc plasma 23 extends only a portion of the distance to the collimating baffle 15 when the gas pressure in the outer regions is maintained at about 10' mm. Hg. In marked contrast to results from normal operation with a reflecting electrode, the potential of the floating electrode 13 reaches a value at least twice the accelerating potential applied to electrons from the filament. Ions in the arc will achieve many times the energy normally expected from normal arc operation. The probe 22 just inside the iron cylinder 21, but just outside the orbit of the most energetic ions, is used, when desired, to record the decay of the plasma when the arc is extinguished. The decay time has been found to vary from 50 to 250 seconds and it varies inversely with the density of neutral gas. As in Fig. 1 the arc of Fig. 2 operates under a pressure gradient and all other characteristics are the same as those described above for linear magnetic fields. Sputtering patterns on the end walls in Fig. 2 after the arc is operated indicate that some of the ions were multiply reflected by the mirrors.

The device of this invention can be used as an ion gun suitable for use in injecting molecular ions into the working volume of a thermonuclear device such as disclosed in the application of John S. Luce, Serial No. 728,754, filed April 15, 1958, or the application of Persa R. Bell et al., Serial No. 753,846, filed August 7, 1958, with energy and density sufficient to accomplish complete ionization and randomization of the plasma. Also, another use of the arc of this invention would be to make it an integral part of the thermonuclear devices of the aforementioned applications wherein the arc is struck along the axis of the devices and accelerates atomic ions directly into the working volume and nearly perpendicular to the magnetic field. The ions are trapped by virtue of their possessing the greater portion of their energy perpendicular to the magnetic field. The are is a virtual source; that is, there are no solid electrodes at the point of ion emission. Therefore, the ions are not lost on their eventual return passage through their origin which is within the confining volume of the thermonuclear devices.

This invention has been described by way of illustration rather than limitation and it should be apparent that the invention is equally applicable in fields other than those described.

What is claimed is:

1. A device for producing energetic ions by transfer of energy from energetic electrons to said ions, comprising a chamber, means for establishing a magnetic field oriented in a selected direction within said chamber, a filament electrode mounted within said chamber, an electron reflector electrode mounted to said chamber in spaced relation to said filament electrode in axial alignment therewith, said electrodes being in axial alignment with the direction of said field, a source of negative elec-, trical potential connected to said filament electrode, means for heating said filament electrode disposed adjacent thereto for causing said filament electrode to emit a copious quantity of electrons which are collimated by said magnetic field to reflux between said insulated reflector electrode and said filament electrode, a pair 'of grounded apertured collimating baffles disposedbetween -5. said electrodes, one each adjacentto each of said electrodes, a gas source, a feed tube disposed in the baffle adjacent to said filament electrode and connected between the aperture in said bafile and said gas'source to 1 inject gas normal to said collimated electrons, evacuating .means connected to said chamber to maintain the presrotating arc discharge about saidcentral arc discharge,

the electrons in said central arc and said rotating are discharge being accelerated by said fields and causing a major portion of said feed gas to be ionized, and a portion of the ions'thus formed being accelerated by said electric fields to an energy equal to at least twice the potential applied to said filament electrode, said ions having a major radial velocity component perpendicular to the direction of said magnetic field.

2. A device for producing an improved gas are discharge, comprising a chamber, means for establishing a selected magnetic field oriented in a predetermined direction within said chamber, electrode means disposed in said chamber for providing an energetic refluxing stream of electrons in alignment with the direction of said field,

evacuation means connected to said chamber for maintaining the pressure therein at a selected low pressure, a source of gas, means connected to said source for feeding gas into the path of said stream of electrons at a constant selected pressure higher than the pressure maintained within said chamber to thereby establish an arc discharge and meaintain a pressure gradient along the thus formed discharge, said pressure gradient establishing rotating time varying, radial electrical fields in the volume surrounding said discharge to thereby cause said discharge to rotate about the arc center line, where- 4. In an evacuated chamber evacuated to a selected low pressue and provided with containing magnetic mirror fields, a heated, negatively biased filament electrode, an electrically'fioating electron reflector electrode, and a pair of grounded apertured collimating bafiies disposed between said electrodes, one each adjacent to each of said electrodes, for collimating a stream of refluxing electrons between said electrodes; improved means for producing very energetic ions by said electrons comprising, a source of gas, means connected to said source for feeding gas at aselected constant pressure into the path of said electron stream and substantially normal thereto to thereby establish anarc discharge along the path of said electron stream, the pressure of said feed gas being maintained at a value higher than the pressure in said evacuated container to thereby provide a pressure gradient along said discharge, said pressure gradient establishing rotating, time varying, radial electrical fields in the volume surrounding said discharge to thereby cause said discharge to rotate about the arc center line,.said rotating discharge including electrons which are accelerated by said electrical fields to an energy at least twice the bias on said filament electrode, said accelerated electrons transferring their energy to the ions in said volume, said energetic ions having a major, radial velocity component perpendicular to the direction of said magnetic fields.

References Cited in the file of this patent UNITED STATES PATENTS 2,764,691 Hippie Sept. 25, 1956 2,772,364 Washburn Nov. 27, 1956 2,826,708 Foster Mar. 11, 1958 2,831,996 Martina Apr. 22, 1958 

